The proposed project deals with the investigation of the fine structural basis of normal and abnormal permeability of the sarcolemmal membrane of the muscle cell. The studies include: 1. light, electron microscopic and cytochemical investigation of normal and increased cellular permeability to tracer substances of different molecular sizes; such as beef liver catalase, cytochrome C, horseradish peroxidase, lanthanum, and the recently developed microperoxidase of Feder. 2. Development of other suitable tracers for use in fine structural studies of permeability in normal and abnormal tissues. 3. Study of endogenous peroxidase activity in muscle and other tissues, to exclude confusion between exogenous tracers with peroxidatic activity (e.g. catalase, horseradish peroxidase, microperoxidase, etc.) and the endogenous enzymes with this type of activity.